Canon 1D X Photo size of this camera should be somewhere in range of 20-30 mb, and with 12 frames per second that sums up to need to write down 300mb/s, or if we consider that the camera has dual card slot, this would amount to 150 mb/s per card (does this work this way?).

Is there anything on market that would sustain such a write speed?

Short: Almost

Longer:

The EOS 1D X has dual CF cards.

Maximum defined CF data transfer rate so far is covered by Revision 6 of the CF standard which was in November 2010 - this added UDMA mode 7 which is rated at 167 MB/s.

What the 1D X will and won't do is still something of a movable feast, but it seems likely that it will offer one or both of dual simultaneous write and interleaved file write options.
IF a CF card capable of maximum UDMA mode 7 was available and IF the camera is able to keep up with dual cards at full speed in interleaved mode, it could write 334 MB/s.
At 12 frames per second that would allow about 334 / 12 = 28+ MB / photo.

MY APSC Sony A77 with 24 megapixels writes 25 megabyte RAW files.

I recall seeing 100 MB/s CF cards announced within the last year. MAY have been LEXAR.
... Gargoyles ...
Yes. Lexar Professional1000X CF - 150 megabytes/second. Available in 16/32/64/128 GB capacities. Hidden in the fine print it says

• Minimum 150MB/s read transfer, write speeds lower.

That is "naughty". I consider that a card,s write speed is it's fair rated speed - that's what the camera cares about when it matters. Looking elsewhere, apparently write speed is 145 MB/s - tolearable :-).

Wikipedia - Compact Flash

Compact Flash Association - surprisingly low in solid information

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XQD:

The Nikon D4 offers dual memory - 1 x CF & 1 x XQD format cards.
Maximum XQD rate so far is 125 MB/s but the card is based on PCIe technology and PCIe 2X and 4X extensions support 250 and 500 MB/s transfers.